Chapter-4 Turbine Regulatory Characteristics and Hydraulic Transients

8/17/2019 Chapter-4 Turbine Regulatory Characteristics and Hydraulic Transients

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CHAPTER -4

TURBINE REGULATORY CHARACTERISTICS ANDHYDRAULIC TRANSIENTS

4.1 TURBINE REGULATING CHARACTERISTICS

Turbine performance characteristics required to be provided for hydraulic transients during start up fromstandstill to synchronizing, load changes and stopping the unit considerably impact design and cost ofhydro stations. These characteristics depend upon design of associated water passage from forebay totailrace and WR 2 of the rotating masses of the unit. Head loss in penstock and pressure water system affectsdirect power loss and optimized by determining economic diameter of penstock and design of bends etc.Pressure and speed regulating characteristics of turbine are required to be provided according to

performance requirement of the hydro electric stations by optimizing pressure water system design andgenerator inertia (GD 2/WR 2).

Simplified method based on Allevis elastic (non rigid) water column theory or using Parmakian’ssimplified methods have been mostly used as illustrated in this chapter. It is recommended that for mega

projects. Proven and applicable computer programmes for calculating pressure changes associated withhydraulic transients for the steady state condition for synchronizing, for partial load changes and those

caused by full load changes caused by transmission line disruption. Hydraulic transients from humanerrors, malfunctioning of equipment, accidents to water conductor systems and earthquakes are taken careof in designing power plant. Indian grid standards (Annexure-1of Chapter 9) require that the frequencyshall not be allowed to go beyond the range 49.0 to 50.5 Hz, except during the transient periodaccompanying tripping or connection of load.

Hydro power plants are being utilized for feeding large grid where precise frequency and voltage controlare required to be provided. Complex interrelated effects between the penstock system including surgetank, turbine, governor, generator and power system requires mathematical simulation of the entire system

be performed for transient and dynamic stability, following a transient period accompanying tripping orconnection of load.

Capability of hydro units for speed or frequency regulation of the interconnected power system cause

hydraulic pressure transients in penstocks which may affect strength (wall thickness), diameter andmaterial. Penstock/pressure water system constitutes significant cost in hydro stations.

Means for controlling pressure and frequency control capability are as follows:

a) Provide additional flywheel effect b) Regulate the opening and closing rate of wicket gatesc) Provide pressure regulatory valves or synchronous bye passd) Provide surge tank

Provision of calibrated rupture membrane in penstock which burst at pre- determined pressure and othermeasures of protection and selection of a Pelton turbine instead of a Francis turbine may also beconsidered.

4.2 Penstock Pressure Regulation

With normal operation i.e. with load accepted or rejected either slowly as the system requires or rapidlyduring faults, pressure water system follow slow surge phenomena and depends upon the rate of closing theguide vanes. The wicket gate closing time is always kept much greater than critical closure time (T c) i.e. the

time of reflection of the pressure wave, this time, T c =al 2

where l is the length of the pressure water

system from tailrace to forebay/ surge tank and a is the velocity of the sound in water (wave velocity).


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4.3 Speed Regulation

The speed regulation or stability of a hydro-electric unit may be defined as its inherent property to ensurethat changes in external conditions as well as in the turbine and governing equipment result an a periodic orrapidly damped, periodic return to the new steady state. Stability over the normal operating range with themachine connected to the system and stability after disconnection can be considered independently. Mosthydro-electric stations are interconnected and as such their satiability is assisted. The more importantfactors upon which the stability of interconnected units depends are the flywheel effect of the unit, the

hydraulic design of the water passages and speed and capacity of the unit. The GD2 should be sufficient to

insure prompt response to power demands and to restrict speed rise following loss of load. But generatorGD 2 should be restricted to avoid excessive power swings. Additional GD 2 built into the generatorincreases the cost, size and weight of the machines and increasing GD 2 more than 50 percent above normaldecreases the efficiency.

Flywheel effect is expressed as starting up time of the unit (T m). This is the time in seconds for torque toaccelerate the rotating masses from 0 to rotational speed

Tm = P

nGD××

×5

22

106.3 (metric units)

Where GD 2 = Product of weight of rotating parts and square of the diameter

n = rotational speed rpmP = Turbine full gate capacity in metric horse power

4.4 Speed Rise

Sudden dropping of load from a unit through opening of the main breaker will cause a unit to achieveconsiderable speed rise before the governor can close the gates to the speed-no-load position. The timerequired to attain a given over speed is a function of the flywheel effect and penstock system. The values ofspeed rise for full load rejection under governor control is considered an index of speed regulatingcapability of the unit. Normally adopted range is from 30 to 60 percent, the former applies to isolated units,where changes of frequency may be important when sections of distributed load are rejected by electricalfaults. Values from 35 to 60 percent are generally adopted for grid connected hydro station. Generally unitsfor which length of the penstock is less than five times the head can be made suitable for stable frequencyregulation of the interconnected system. Also units for which Tm ≥ (Tw) 2 can be expected to have goodregulating capacity. This test should be applied over the entire head range. Plants in which more than oneturbine are served from one penstock should be analyzed to determine proper governor settings andappropriate operating practices. Such plants may be unable to contribute to system transient speedregulation but adverse effects upon the system may be avoided by specifying the number of units whichmay be allowed to operate on free governor (unblocked) at any one time.

The turbine and generator are normally designed to withstand runaway speed, but at excessive speed severevibrations sometimes develop which may snap the shear pins of the gate mechanism. To minimizevibration, a speed rise not to exceed 60% can be permitted in contrast to the 35 to 45% desired forsatisfactory regulation of independently operated units.

Pressure Rise and Speed rise Considerations as per IS: 12837.This criteria is more important in high head machines as higher pressure rise affects the cost of penstocksubstantially. Necessity of limiting pressure rise is accomplished by use of pressure relief valve in case ofKaplan and Francis turbine with relatively, long water conductor system resulting in increased cost ofequipment and power house. Pelton turbine as a rule does not require this device on account of availabilityof design feature of deflector. Pressure rise and speed rise can therefore be limited to very economical levelin case of Pelton turbine, without increase in cost of turbine. Permissible pressure rise and speed rise forvarious turbines are given below.


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Type of turbine Pressure rise (%) Speed rise (%)Pelton 15 to 30 20 to 45Francis 30 to 35 35 to 55Kaplan/bulb and Propeller 30 to 50 30 to 65Deriaz 20 to 45 35 to 65

4.5 Considerations for Permissible Speed Rise on Full Load Rejection

4.5.1 Generator flywheel effect and Stability of Turbine Governor System

Large modern hydro generators have smaller inertia constant and may face problems concerning stability ofturbine governing system. This is due to the behaviour of the turbine water, which because of its inertiagives rise to water hammer in pressure pipes when control devices are operated. This is in generalcharacterized by the hydraulic acceleration time constants. In isolated operation, when frequency of thewhole system is determined by turbine governor the water hammer affects the speed governing andinstability appears as hunting or frequency swinging. For interconnected operation with a large system thefrequency is essentially held constant by the later. The water hammer then effects the power fed to thesystem and stability problem only arises when the power is controlled in a closed loop, i.e., in case of thosehydro generators which take part in frequency regulation.

The stability of turbine governor gear is greatly affected by the ratio of the mechanical acceleration time

constant due to the hydraulic acceleration time constant of the water masses and by the gain of thegovernor. A reduction of the above ratio has a destabilizing effect and necessitates a reduction of thegovernor gain, which adversely affects frequency stabilization. Accordingly a minimum flywheel effect forrotating parts of a hydro unit is necessary which can normally only be provided in the generator.Alternatively mechanical acceleration time constant could be reduced by the provision of a pressure reliefvalve or a surge tank, etc., but it is generally very costly. An empirical criterion for the speed regulatingability of a hydro generating unit could be based on the speed rise of the unit which may take place on therejection of the entire rated load of the unit operating independently. For the power units operating in largeinterconnected systems and which are required to regulate system frequency, the percentage speed riseindex as computed above should not exceed 45 percent. For smaller systems smaller speed rise be

provided.

4.5.2 Large Units (grid connected)

(a) Frequency controlling hydro station(b) Non frequency controlling station

Design requirements of frequency controlling station are availability of water to meet fluctuatingrequirements e.g. dam, balancing storage and speed regulating capability. Speed rise for full load rejectionof these units may vary from 35 to 45% depending size of the grid with respect to the unit. Speed rise onfull load rejection provided for some of the frequency controlling stations are given below.

SlNo.

Unit Size Grid Speed rise on fullload rejection

1. Ganguwal/Kotla units ofBhakra Nangal (1950)

24,000 kW Northern Grid (small) 33.5%

2. Bhakra Power Plant (1958) 100,000 kW - do - 35%3. Dehar Power Plant (Beas

Satluj Link Project) 1975165,000 kW Northern Grid (large) 41%

4. Pacha SHP 1,500 kW Isolated/weak grid 35%

Frequency control generating unit governor characteristics will depend on grid characteristics as follows:

(a) Sensitivity i.e. speed change to which governor will respond may be 0.01% or less for large gridswith thermal nuclear stations forming part of grid. This may be evaluated for the particular grid.

(b) Permanent speed droop setting should be 0-5%.


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(c) Temporary speed droop, servomotor feed back time etc. should be suitable for grid. National gridrequirement are given in Annexure 1 of Chapter 9).

(d) It is recommended that stability studies be carried to fix these settings parameters for frequencycontrolling stations.

(e) It is further important that efficiency operations of the units be ensured for this purpose frequencycontrol stations governors should be properly equipped to analyse efficiency loading of the unitsand load on the units is made by changing speed level (speed at no load) for moment to momentload changes by automatic means. This will need digital governors as in manual control power

stations load change on the units takes place by permanent speed droop settings and speed level isadjusted manually.

4.6 Non Frequency Control Station

Non frequency control grid connected stations having storage of water are equipped for efficiencyoperation for moment to moment grid load changes by digital slow governors by transferring load fromfrequency controlling stations which will take instant to instant load changes and transfer them to nonfrequency control station where water is available. The station is equipped for automatic load frequencycontrol arrangement for optimum efficiency operation. It may be noted that in these stations the loadchange will occur not for frequency control but for efficiency operation. In Pong Power Plants surge tankhad to be eliminated; very slow governor (large governor opening/closing time) and water saving type

pressure regulators were employed to reduce penstock stresses for on load rejection and for synchronizing

unit. The unit was non frequency controlling as critical regulation occurred during load on conditions.

4.7 Small Hydro (grid connected)

Small hydro if grid connected (with no isolated and or islanding provision) cannot take part in frequencycontrol. Accordingly these may be designed for up to 60% speed rise on full load rejection. In canal fall orsimilar units, speed control is required only during synchronizing. Generator loading should be controlled

by level i.e. non speed control governors are used and loading on the units is controlled by upstream canalwater level by controlling gate limiters. These are called non speed control governors.

4.8 Small Hydro (isolated grid operation)

These are designed as frequency control units for the criteria that speed rise on full load rejection do not

exceed 35%.

4.9 Micro Hydro

Micro hydro up to 50 kW unit size are controlled by electronic load controllers instead of a traditionalgovernor. This avoids the need to change water flow through the turbine as the electrical load changes.Water heater with variable resistance (dump load) is connected to the generator. As the electrical systemload required from the generating unit changes electronic controller changes the dump load. Generatoroutput and corresponding turbine flow remain constant. Micro hydro fitted with conventional water flowgovernors are designed for 35% speed rise on full load rejection.

4.10 Performance Characteristics - Pressure Rise and Speed Rise Calculation

The penstock pressure rise and unit speed rise are calculated from the references given in Para 4.2 entitled‘penstock pressure regulation’. Preliminary economic studies are required to be carried out to determinewhether more than normal GD 2, a larger penstock, a surge tank or a pressure regulator is required. Methodof calculating based on USBR design Monograph No. 20 for a typical power plant is given below.Penstocks for the power plant were embedded for a maximum pressure rise of 28%. The entire power wasto be fed into the grid allowing a speed rise on full load rejection up to about 60%.


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Method for Computing Penstock Pressure Rise and Speed Rise on full load rejection with assumeddata

DataTf = Servomotor minimum closing time, sec. – 5 sec.Pr = Turbine full gate capacity of hr, kW – 92.6 MW (Gen. terminals)hr = Rated head, meter – 58.5 mn = Rotational speed: design, r/min. – 166.6 rpm

GD2 = Flywheel effect of revolving parts; kgm

2 – 7.068 x 10

6 kgm

2

L = Equivalent length of water conduit, m – 130.7 mA = Penstock diameter – 5600 mmQ = Full gate turbine discharge at rated head – 183.1 m 3/sec.V = velocity of water (Q/A) – 7.4 m/sec.

N s = Specific speed – 320 metric horse power unitTm = Mechanical startup timeTw = Water startup time

Pressure rise on full load rejection for closing Time (Allievies Formula)

( )42

2 ++=∆ nnn H H

Where n = gHT LV

L - Length of penstock + ½ the length if the spiral casing =130.7 +16.9 = 146.6 mH – Head in mT – Governor closing time in secondsV – Velocity in m/sec.g = 9.81 m/sec 2

This formula is sufficiently accurate only of T >a L4

where a is the wave velocity.

Note – Use plus for pressure rise and minus for pressure drop.Minimum Governor closing time to be determined (4 to 8 sec)

n = gHT LV

=T ××

×5.5881.9

4.76.146=

T 89.1

For governor closing time of 5 sec.; n = 0.378

Pressure rise in penstocks H H ∆

is as follows:

( ) ++=∆

4378.0378.02378.0

2 H H

= 0.456 = 45.6%

Speed rise on full load rejection for 5 sec. closing Time (Based on USBR monograph no. 20)

TW = water start up time =GH LV

= 1.89 sec


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Fig. 4.1 – Turbine Performance(Based on USBR Design Monograph no. 20)


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4.11 Mega and Large Units - Turbine Governing, Speed and Pressure Regulation, Relief Valves,Frequency Control Economic Considerations

Regulating characteristics, speed rise (penstock pressure rise) of hydroelectric plants are discussed in 4.10.Basis for determination when surge tanks (or pressure relief valves) will be required on turbine penstockinstallations are discussed with special reference to Bhakra Beas Complex (Para 5.1-Overview).

4.11.1 Bhkra Left Bank – Frequency Controlling Station

Variable head Bhakra Left Bank Power Plant commissioned in 1959-60 comprises 5 units of 90 MW eachand was the first mega power project of the country. Each unit was fed by 4.572 m dia. (15 ft.) penstock.

Relevant Data

5 sets of 150,000 BHP (Gen. 90,000 kW+ 15% overload)

Rated head – 400 ft. (121.9 m)

Rated flow – 3610 cusecs for fullgate opening at rated head

Speed - 166.7 rpm

WR 2

(flywheel effect) - 63 x 106

lbs ft2

normal for each unit

Length of penstock - 750 ft.(individual)

Penstock diameter - 15 ft.

Specific speed of runner - 36 (fps units)

The maximum demand of the power system to which the plant was interconnected at that time hadmaximum demand of about 1000 MW and minimum demand of about 500 MW. Speed rise on full loadrejection as an index of regulating ability of the unit was fixed as 35% on full rejection. These units beingfed from the storage dam ideally suited for regulating system frequency or peak load operation. Governorclosing time of the mechanical governor was fixed from following considerations.

i) Speed rise on full load rejection not to exceed 35% for satisfactory operation as the frequencycontrol station of the system.

ii) The generating units are rated to give a normally continuous output of 125,000 HP by the turbineand overload rating of generators required 150,000 HP turbine output. Maximum pressure in

penstock and speed rise on full load rejection to be determined for overload rating.iii) Retardation of governor closing time above rated head was not considered desirable.

A minimum governor closing time of four seconds was found to be suitable for this purpose. Speed rise and pressure rise for rated head and above is given below.

i) Head on turbine (h) 121.9 m(400 FT.)

146 m(480 FT.)

156 m(512 FT.)

REMARKS

ii) Gate opening for anoutput 150,000 HP

Approx. figure as perHitachi (turbine supplier)

100% 72% 65% 150,000 HP isover load rating

iii) Actual governor closingtime (equivalent closingtime of guide vanes)

Overall closing time 4seconds including 0.3sec. as dead time

3.7 x % gate opening

100

3.7 sec. 2.66 sec. 2.4 sec. Assuming time ofclosure from any

part gate opening asdirectly proportionalto the time ofclosure from fullgate opening


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iv) Pressure rise in

penstock )( H ∆ 38% 36% 34%

v) Max head on theturbine including waterhammer

168.25 m(552 ft).

198.1 m(650 ft.)

208.8 m(685 ft.)

SAFE

vi) Speed rise S R 35% 27% 25%

Penstock designed for 35% penstock pressure rise on full load rejection at maximum head. 4 second governor timefor load on or off was provided.

4.11.2 Large hydro Bhakra Right Bank Power Plant with frequency controlling, capability, increase in unitsize on already embedded penstock by providing extra flywheel effect

Bhakra Right Bank Power Plant was also planned for installation of 5 units of 90 MW each (same asBhakra Left Bank Power Plant). Five No. of penstock were laid accordingly. Before construction of the

plant, a new scheme Bhakra Beas Satluj Link project was envisaged. This project was intended to divertriver Beas water into River Satluj upstream of Bhakra Dam. Increased supply of water and need foradditional peaking capacity at Bhakra necessitated increase in capacity and size of the generating units to120 MW at Bhakra Right Bank (Bhakra Left Bank was already constructed). Penstock pipes were alreadyembedded for 90 MW unit size. The power plant was to be interconnected with Northern Regional grid andwould be largest unit size in the then grid. It was decided that the power house will be designed as afrequency controlling station. For this purpose, the criteria of keeping speed rise on full load rejection of35% as the index was decided to be maintained. It was further decided that the maximum stresses in the

penstock will not increase 34% at maximum head under governor control. This was achieved by increasingthe governor closing time of the power plant units of 120 MW to 6 seconds and increasing generatorflywheel effect above normal. Salient data of the water conductor system was the same as for Bhakra LeftBank

Data

Rated capacity of the unit was increased from 90 MW to 120 MW and Governor closing time was fixed sixseconds so that stresses in the already embedded penstock do not increase original design value (4.11.1).The turbine designer fixed the rated speed at 187.5 rpm.

To keep speed rise on sudden throwing off of load at 35% at rated head and maximum stresses in penstocknot to exceed 34% on maximum head was calculated as 6 seconds.

Required minimum flywheel effect of rotating parts of unit was calculated as follows:

GD 2 = 16.1 x 10 6 kg m 2 (WR 2 = 95 x 10 6 lb. ft. 2) for satisfactory speedregulation (for a speed rise of 35%).

The normal flywheel effect of a hydro-electric unit was computed from the following empirical formulae(USBR Engineering monograph no. 20-1954):

(i) Generators

( )2

4/5

2/32 .

..000,379 ft lb

m pr kVA xWR

= …(1)

where kVA = Generator rating in kVA. Normal flywheel effect of modern generator is less and may be taken as

( )2

3/5

2/3 ...000,356 ft lb

m pr

kVA x


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(ii) Turbine

WR 2 =( )

24/5 ...

..23800 ft lb

m pr

ph x

…(2)

Where h.p. = Turbine rating at rated head (full gate)= 170,000 h.p.

Normal GD 2 OF GENERATOR AND TURBINE= Eq. (1) + Eq. (2)WR 2 = 63 x 10 6 lb. ft 2 (GD 2 = 10.67 x 10 6 kg. m 2)

It is obvious that about 45 – 50% additional flywheel effect will have to provide in the unit for satisfactoryregulating characteristics of the unit for the same speed of the unit.Effect of Extra Flywheel

Extra loading besides affecting the cost, size, weight and efficiency of the unit, may also have a bearing on thestability of the unit as unstable unit, then either unit size may have to be modified or else the need of pressure reliefvalve studied.

Power Swing

Power swing is a periodic transfer of load between machines but not affecting the system as a whole. These swingsare in step with the draft tube surges and have a frequency equal to one-third of the revolution per minute of the unit.They may build up to an amplitude that will cause the main breaker to trip and take the unit off the line.

Experience has shown that power swings be expected if the maximum regulating constant (kd) increases the limit(USBR Designed Standards).

Maximum kd = 5,400,000( )

4/1

2/3.m. p.r

kVA

Where kd, the regulating constant, is

kd =. p.hDesign

.)m. p.r (xWR Total 22 …(4)

Where total WR 2 = turbine WR 2 (normal) + generator WR 2 (normal) + loading (additional generator WR 2 abovenormal

Evidently from (4) above kd is increased if WR 2 increases when actual WR 2 installed is in excess of the powerswing limit, the severity may be sharply reduced through the use of a small amount of air admitted to the draft tubeand fins or guides in the draft tube just below the runner to reduce whirl and surges. For the example referred toabove:

Maximum kd = 5,40,000 x( )

4/1

2/35.187

000,134

= 14.5 x 10 6

Actual kd =( )000,170

5.1871095 26 x x

= 12.1 x 10 6

Thus it is expected that with a flywheel effect of GD 2-16.1 x 10 6 (WR 2-95x10 6 lb ft 2), stable operation would bethere and there would be no undesirable power swings.


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It may be conclude as follows:(i) Speed rise of not more than 35 percent on sudden load rejection is required for controlling stations in weak

grids/small grids.(ii) Governor time (overall) from full gate to no load at normal speed when increased so as to restrict stresses in

penstocks may result in increasing the flywheel effect of the revolving parts of the machine (generator) forfrequency controlling stations.

(iii) The extra loading thus provided so as to increase the flywheel effect may affect the stability of the unitwhich should be studied.

4.11.3 Pong Power Plant (6 x 60 MW) on Beas Dam – Elimination of surge tank

Rated head 64 m (210 ft); maximum head 95.5 m (310 ft); minimum head 46 m (156 ft); rated discharge157.64 m 3/s (5600 cusecs); velocity at rated head 4.7 m/sec. (15.6 ft/sec); penstock – diversion tunnelswere converted into pressure tunnel and bifurcated to feed two units with following characteristics.

Portion of

tunnel/penstock

1

Concrete

2

Steel

3

Steel (Branch)

Common header for 2 units

Length m (ft) 288.7 (947 ft.) 335.8 (1103 ft.) 51.8 (170 ft.)

Diameter m (ft) 9.1 (30 ft.) 7.3 (24 ft.) 5.18 (17 ft.)Thickness m (ft) 60 (2.5 ft.) 40 (0.133 ft.) 33 (0.108 ft.)

Mean velocity at rated head 4.7 m/sec. (15.6ft/sec) Length of the penstock L and rated head ratio is

64

676

2102220

= 10.5. This is more than 5 A power units for stable speed regulation will normally

require a surge tank or pressure relieve valve for stable system frequency control. It was considered during planning stage that provision of surge shaft is not feasible due to economic and other considerations.Further pressure relieve valve of water wasting type can not be employed. Accordingly it was decided tomake it a non frequency control station with provision of water saving type pressure regulator forsynchronizing and for containing stresses in the penstock under normal condition. Studies carried outmanually and are summarized in table 4.2 & table 4.1. Water hammer studies for pressure rise in penstockfor governor closing times of 6 to 10 seconds for full load throw off for various conditions i.e. normal,emergency and abnormal as defined in table 4.2 without pressure relief valves and with pressure reliefvalves of 50% and 100% are given in table 4.2. Speed rise on full load rejection studies are summarized intable 4.1. Studies for governor closing time less than 6 seconds were not carried out.

After more detailed analysis and considerations for safety of water conductor system by supplier ofequipment (BHEL), it was decided to install 100% pressure relief valves with following maincharacteristics.

Type : cylindrical balanced typeDischarge capacity at rated head : full discharge

Method of operation: oil operated servomotors coupled to governor servomotorTime of opening : same as guide vane openingMaximum pressure rise : 23%

Governor opening time: It may be noted that water saving type pressure regulator were installed.Governor opening time was fixed 12 seconds. Critical regulation occurs during load on conditions and


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accordingly operation of the powerhouse was restricted as per Para 4.6. Generator flywheel effect GD 2 =8.45 x 10 6 kg m 2 (WR 2 – 50 x 10 5 lbs ft 2+) was provided. Maximum gross head on turbine is123.8 m.

TABLE -4.1: PONG POWER PLANT

Speed Rise on 100% Load Rejection At rated Head of 210 ft.

Sl.No.

Operating Conditions Speed risepermissible

%

Speed Rise %

WithoutRegulator

50% capacityRegulator

100% capacityRegulator

6 sec 8 sec 10 sec 6 sec 8 sec 10 sec 6 sec 8 sec 10 sec1. Normal Operation

(i) One unit on a common penstock working

451 47 56 63 38 47 56 32 41 50

(ii) Both units on a common penstock working

602 60 68 75 47 56 63 - - -

2. Emergency operation(One Pressure Regulator fail)(i) Both Units working 60 2 50 59 67 47 56 63

3. Abnormal operation

(Both Pressure Regulator fail)(i) Both Units working Runaway 60 68 75 60 68 75

Notes:1. Speed rise are with maximum GD 2 of 8.45 x 10 6 kg. m 2 (WR 2 of 50 x 10 6 lbs ft 2)

2. (1) Used as an index of speed regulating capability of the units

3. (2) Used from mechanical safety considerations

TABLE -4. 2: PONG POWER PLANT

Water Hammer (Pressure rise) Maximum operating head = 94.5 m (310 ft.)

Sl.

No.

Operating

Conditions

Max. Head attained including Water Hammer Heads

6 sec. 8 sec. 10 sec.W/o R* 50% R 100% R W/o R* 50% R 100% R W/o R* 50% R 100% R

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.1. Normal 186.5 m

( 261 ′ )126.5 m( 541 ′ )

116.13 m( 138 ′ )

153.6 m( 250 ′ )

115.2 m( 837 ′ )

116.1 m( 138 ′ )

135.3 m444 ′ )

140.34 m( 236 ′ )

116.1 m( 138 ′ )

2. Emergency 222.5 m(730)

198.1 m(650)

182.9 m(600)

185.9 m(610

170.7 m(560)

160.3 m(526)

164.8 m(541)

153.9 m(505)

146.3(480)

3. Abnormal 271.2 m(890)

271.2 m(890)

271.2m(890)

225.8 m(741)

225.5(740)

225.8 m(741)

198.1 m(650)

198.1 m(650)

198.1 m(650)

4. Extreme 509.0 m(1670)

509.0 m(1670)

509.0 m(1670)

509.0 m(1670)

509.0 m(1670)

509.0 m(1670)

509.0 m(1670)

509.0 m(1670)

509.0 m(1670)

* R means pressure regulationNotes:

1. Water Hammer was calculated for full load rejection on both units simultaneously.2. Operating condition were assumed as below:(i) Normal operation: when all equipment works in the manner for which it is designed & adjusted.(ii) Emergency operation: following maladjustment & malfunctioning of equipment occurs on one of

the units.

a. Governor time remains un retarded b. Cushioning stroke fails i.e. gate closure from 2L/a position.c. Pressure relief valve if provided is in-operatived. Gate traversing is taken in min. time for which the governor is designed.


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(iii) Abnormal Operation: Malfunctioning of equipment as described under emergency and occurs on both the units on a common penstock.

(iv) Extreme Operation: there is a rapid closure of the gates and full velocity in the penstock comes tozero within the critical interval. Calculation made at rated head of (64.1 m) 210 ft

a) Gate opening time 10 second (max. 12 seconds) b) Minimum speed change to which

governor will respondGovernor will respond to normal speedchanges; however response will be inhibited

by adjustable bandc) Gate closing time in conjunction with

pressure relief valves3.2 seconds –rate of closure excludingdamping

Electro Hydraulic Governor was provided.

4.11.4 Dehar Power Plant With Frequency Controlling Capability, Provision of Surge Shaft Tank AndBalancing Reservoir

174 MVA, 300 rpm 0.95 p.f. semi umbrella type vertical water wheel generators coupled to Francisturbines were selected for installation in the 1000 MW dehar hydropower plant of the Beas Sutlaj linkProject. A longitudinal section of the project is shown in figure 4.2.

Dehar power plant was the largest hydro project at the time of installation and constituted a significant plant capacity in the integrated system to which the power house was to be interconnected. Salient data ofthe project is given below.

Unit size and No. : Six units each 165 MW at 0.95 pfDesign head : 320 m

No. of penstock : 3Diameter of penstock header : 4.88 mDiameter of penstock branch : 3.35 m

Surge Shaft – Differential TypeDiameter of main shaft : 22.86 mDiameter of riser shaft : 7.62 mHeight : 125.3 m

It was decided to make a frequency control power station by providing balancing storage and a differentialtype surge tank. A maximum speed rise and full load rejection as an index of frequency control capabilityin the grid was fixed not to exceed 45% (as the grid had become fairly large). The penstock pressure risefor economic considerations was fixed 35% on the basis of both the units tripping on a common penstockheader. Load on and load off in about eight seconds was considered satisfactory from system regulationconsideration.

Hydraulic pressure water system connecting the balancing storage with the power unit consisting of waterintake, pressure tunnel, differential surge tank and penstock is shown in Figure 4.2.

Limiting the maximum pressure rise in the penstocks to 35 percent the estimated maximum speed rise ofthe unit upon rejection of full load worked out to about 45 percent with a governor closing time of 9.1seconds at rated head of 282 m (925 ft) with the normal flywheel effect of the rotating parts of thegenerator (i.e., fixed on temperature rise considerations only). In the first stage of operation (4 units) thespeed rise was found to be not more than 43 percent. It was accordingly considered that normal flywheeleffect is adequate for regulating frequency of the system.


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Fig. 4.2: Longitudinal Section from intake to Dehar Plant(Design Data)

4.12 Small Hydro Projects

Small hydro projects are designed for 35% speed rise in case isolated operation is envisaged. Cost is themajor consideration in designing the powerhouses. Canal fall schemes are generally designed for injectingthe entire power into the grid and accordingly designed for 60% speed rise on full rejection and cannot

provide frequency control of the interconnected power system. Micro hydrohydro is generally designed forisolated operation. In small hydro projects extra flywheel required is not built into the generator but providedas a separate flywheel. Some typical examples are given.

4.12.1 1750 kW power unit to be designed for isolated operation (35% speed rise)

Data

Length of Penstock (L) = 153.5 mPenstock Dia (D) = 1.289 mPenstock thickness = 0.00889 m = 8.89 mmRated unit output = 1750 kW (including 10% over load capacity)(full gate) (1750 x 1.34 = 2345 HP units)

Rated Head (h) = 46.634 m(Full gate)

Maximum pressure rise = 30%in penstock fromeconomic consideration

First Step: - Fix closing time for 30% speed rise

Assuming governor closing time of 4 seconds

Rated Discharge (Q r ) =8.0804.9 ××r h

P

=8.0804.963.46

1750××

= 4.78 cusecs


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Velocity of water (V r ) = Q/A (A – cross sectional area of penstock)

=( )2289.14/78.4

×π =

661521.17854.078.4

×

= 3.662 m/sec.

Governor closing time (assumed) = 4 second

Guide vane closing time assuming (t 0) = 4 + 0.25 = 4.25 second(0.25 sec. as dead time)

Gravitational Constant (g) = 9.81 m/sec 2

Water starting up time (Tw) = gH LV

=63.4681.966.35.153

××

= 1.228 second

Pressure rise on full load rejection using Alliivies formula

H H ∆

= { }42

2 ++ www T T T

Where T w = gHT LV

=25.4

2287.1 = 0.2894 = 0.29

L = Length of penstock + Length of Spiral Casing = 153.5H = Head in meter = 46.63T = Governor closing time 4 secondsV = Velocity in meter/second = 3.66 m/s

g = 9.81 m/s2

H H ∆

= { }429.029.0229.0 2 ++

= 33.50%Speed Rise and WR 2

Normal WR 2 of Gen. & Turbine 42000 lb/ft 2 (GD 2 = 7 Tm 2)

Mechanical starting up time Tm =r P

nGD××

×5

22

106.3 =

1750106.3750107

5

23

××××

= 6.25 seconds

Closing time of servo motor T f = 4 seconds (full closing time of servomotor)

m

k

T T

=2.6

4 = 0.645

Specific speed n sr = 4/5h P n

= 4/563.461750750

=48.121751.31374

= 257.48 = 258 (m units)

Speed rise S r = 26.5% (from figure 4.1)


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Tw = 1.23

k = f

w

T T

=423.1

= 0.3075

S'R = (26.6) (1 + 0.3075)= 34.779 = 34.78%

4.12.2 Rajwakti Small Hydro-Electric Project – Provision of Spilling Type Surge Tank for Economic

Pressure Water System

A run of the river project in cascade with downstream projects. Entire power is to be fed in the grid at 66kV. Isolated operation was not envisaged. Entire tail race water was proposed to be provided fordownstream projects in cascade. Frequency regulating capability was not required. Due to site limitationsof locating forebay the penstock length was unusually long i.e. 980 m for head of about 47 m which wasaffecting the viability of the project. Further it was not possible to provide escape of water from the forebayon full load rejection for downstream project. The problem was solved by providing spilling type surgetank near the powerhouse. It is shown in figure 4.3(a) and figure 4.3(b). The spilling type surge tank andthe penstock system are shown in figure 4.4. Relevant data of the powerhouse and spilling type surge tankis given below. A water level sensing system was installed in forebay to control the spilling level in thesurge tank.

Design head = 46.65 mPenstock (L) = 980 mPenstock dia. (D) = 2.2 m common penstock header bifurcated near

power house for each unit

Turbines = Horizontal axis, FrancisSpecific speed = 218Rated output = 1975 kWRotating speed = 600 rpm

Generator = 2250 kVA, 3.3kV, 0.9 pf, 600 rpm (Synchronous)Surge LineDiameter = 2.2 mLength = 100 mInput height = 933.9 + 2.2 mOutput height = 983.115 mDesign flow = 10 m 3/sec.

Surge tankBottom level = 983.115 mTop level = 979.45 mWidth = 6.5 mLength = 11.5 m

Spilling Pipes No. of spilling pipes = 2Bottom level = 975.45 mTop level = 6.5 mDesign flow = 0.70 m 3/sec

The pipe line from forebay to the surge tank was designed for static head.

Solution

i) Spilling type surge shaft was providedii) Penstock header designed as pipeiii) Spilling level was controlled by level control in forebayiv) Spilling water from surge tank near the powerhouse was channelised into tailrace for downstream

powerhouse in cascade


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Spilling TypeSurge Shaft

Unit-1

Unit-2

Forebay

Pipe

Figure 4.3 (a) : Proposed Arrangement

FOREBAY

PENSTOCKPIPE

BUTTERFLYVALVE

BUTTERFLYVALVES

SURGE TANK

EXPANSIONLINE

SPILLINGPIPES

STILLINGTAILRACE

DRAFT

FRANCISTURBINE

GENERATOR

TO DIVERSION &POWER CHANNEL

WATERESCAPE

TO UNIT-2

PIPE LINE LENGTH - 980 M

PIPE DIA - 2.2 MDISCHARGE - 10 CUMECS

SURGE TANK

SPILLING CAPACITY

70%OF 10CUMECS

Figure 4.3(b): Rajwakti SHP – Spilling Type Surge Tank (Source: Alternate Hydro Energy Centre))

Fig.: 4.4: Spilling type surge tank and penstock system (Source: Alternate Hydro Energy Centre)


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4.12.2 Pacha Small Hydropower Project Designed for Isolated Operation

Layout of the project is shown in figure 4.5.

4.12.2.1 Data

Type of Turbine : Horizontal FrancisGuaranteed rated output at rated head : 1571 kW

Rated head :Rated speed : 750 rpmSpecific speed : 243.7 m-kWLength of penstock : 138.9 mIndividual penstock for each unitPenstock diameter :Max. Pressure rise for economic penstock : 30%Momentary rise in speed on full load rejection : 35%Pressure rise and speed rise studies were made for 600 rpm and 750 rpm generators (table 4.3 & table 4.4).750 rpm generators were selected from economic considerations.

Fig. 4.5: Layout of Pacha Small Hydro Project(Source: Alternate Hydro Energy Centre)


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Table 4.3Speed 750 rpm

S.No.

Speed rise Pressure rise Guide vane closing time Required GD 2

1. 35% 30% 7.2 sec. 11.05 T-m 2

2. 35% 50% 4.7 sec. 8.4 T-m 2

3. 40% 45% 5.1 sec. 7.35 T-m 2

Table 4.4Speed 600 rpm

S.No.

Speed rise Pressure rise Guide vane closing time Required GD 2

1. 35% 30% 7.2 sec. 17.30 T-m 2

2. 35% 50% 4.7 sec. 13.15 T-m 2

3. 40% 45% 5.1 sec. 11.50 T-m 2

4.13 List of Computer Programmes on Hydraulic transients.

References

1. Hydro Electric Engineering practice by J. Guthrie brown (Book)

2. Engineering Monograph No. 20 – 1976 -Selecting Hydraulic Reaction Turbine – United StateDepartment of Interior; Bureau of Reclamation; USA

3. ASME -1949, Rober Lowy – Speed Regulation Characteristics for Hydraulic Turbine

4. John Parmakian – 1957 – Paper No. 1216 Journal of Power division of ASME vol. 83.

5. O. D. Thapar & S. M. Kumar – A problem of speed regulation in hydro electric units – Power engineer1962 Vol. 12 pp. 31-34.

6. Thapar O. D.; V. K. Verma; R. C. chauahan – 1980 “Bhakra Left Bank Power House Governor - Studyof Behavior – Engineering report No. WRD – Electrical -07- WRDTC IIT Roorkee

7. Thapar O. D. – 1975 “Characteristics of Large Hydro Generator of Dehar Power Plant” – ProceedingSecond World Congress International Water Resources Association, New Delhi – December 1975Volume 1.

8. Alternate Hydro Energy centre – “Guidelines for Selection of Turbine and Governing System forSmall Hydro – Electric Project – 2012.

Chapter-4 Turbine Regulatory Characteristics and Hydraulic Transients

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